Vapor phase systems are known in which a processing vapor is contained in a vessel and into which a product is introduced to accomplish a particular process or operation. One such system is for vapor phase soldering wherein solder on a workpiece is caused to melt or reflow by the heat from a condensing heated vapor. In such a vapor phase soldering system, a fluorocarbon liquid is typically provided in a tank and is heated to a temperature sufficient to produce a hot saturated vapor above the surface of the liquid and in equilibrium therewith. This liquid preferably is a nonconducting chemically stable inert liquid having an atmospheric boiling point at least equal to the soldering temperature and which provides non-oxidizing, non-flammable vapor. The tank is usually open to the atmosphere to facilitate product entry and removal, and the vapor can emanate from the open tank into the atmosphere. The release of process vapor into the atmosphere can present a health risk, and the magnitude of such emissions are becoming the subject of increasingly stringent governmental and industry health and safety standards. Moreover, the liquid providing the vapor is relatively expensive, and thus for economy of system operation, loss of the liquid through vapor emission into the atmosphere should also be minimized.
One technique for minimizing vapor loss is shown in U.S. Pat. No. 3,904,102 wherein a secondary vapor blanket is provided over the primary or processing vapor to shield the primary vapor from the atmosphere. The secondary vapor blanket is most typically formed of Freon TF (R-113) which stratifies above the primary vapor by virtue of a lower boiling point and a lower density than the primary vapor. Such a dual vapor system presents several disadvantages. The system is inherently more complex than single vapor systems by reason of the additional cooling and recovery equipment required for the secondary fluid. The secondary vapor is not in contact with its own boiling phase but rather is exposed to the higher temperature primary vapor; thus, the second vapor is caused to exist at a superheated temperature, usually about 180.degree.-225.degree. F., which causes breakdown of the material into components which can be toxic and corrosive. Acid contaminants can form in the secondary vapor, which can affect the processing system and add to the cost of its maintenance. The presence of acid contaminants in the secondary vapor can also result in contamination of the product entering and leaving the system by way of the secondary vapor blanket. Such acid contaminants exist in the secondary vapor to some degree even when employing acid removal procedures on the condensed secondary liquid. Moreover, the boiling phase of the primary liquid can generate pollutants such as perfluoroisobutylene (PFIB) which can enter the atmosphere.
Systems have been proposed using tank covers to contain the processing vapor, however, vapor loss still occurs when the cover is opened to admit product entry and removal. An improved system for minimizing the problems of vapor loss is shown in U.S. Pat. No. 4,077,467 of one of the same inventors and the same assignee as herein. In the system therein described, the primary and secondary liquids are separated and contained within closed individually controlled vapor-lock chambers. The separate chambers prevent the vapors from communicating with each other, inhibit the flow of contaminants, minimize vapor loss and avoid fluid breakdown due to excessive heating. The separation of the chambers is achieved by movable doors which sequentially open to admit the work and close behind the work; after the soldering is completed, the doors operate in reverse sequence. However, this added equipment complexity somewhat hampers the vapor phase soldering operations in an automated manufacturing facility. Furthermore, to a lesser extent, some of the above-mentioned vapor loss problems persist.
A system showing the continuous passage of articles into and out of the vessel containing the vapor is shown in U.S. Pat. No. 3,866,307, wherein the process vessel includes an inlet conduit and an exit conduit, each disposed at the top of the vessel above the vapor zone, and each having cooling coils which assist in retaining the vapor within the vessel. Articles to be processed are conveyed in a curved path downward from the inlet conduit into the vapor and upward from the vapor to the outlet conduit. This apparatus has application to manufacturing processes, generally limited to where the vapor entry and exit and curved path are permissible. Vapor loss can also occur from the relatively large inlet and outlet ports.